Slot assisted grating based transverse magnetic (TM) transmission mode pass polarizer

ABSTRACT

The present disclosure relates to semiconductor structures and, more particularly, to a slot assisted grating based transverse magnetic (TM) pass polarizer and methods of manufacture. The structure includes: a waveguide strip composed of a first type of material and having openings along its length which are positioned to reflect/scatter a propagating electromagnetic waves; and grating fin structures on one or both sides of the waveguide strip which are positioned and structured to reflect/scatter the propagating electromagnetic waves.

FIELD OF THE INVENTION

The present disclosure relates to semiconductor structures and, moreparticularly, to a slot assisted grating based transverse magnetic (TM)pass polarizer and methods of manufacture.

BACKGROUND

A waveguide guides waves such as electromagnetic waves through a mediumwith minimal loss of energy by restricting its expansion. Without thephysical constraint of a waveguide, wave amplitudes decrease accordingto the inverse square law as they expand into three dimensional space.Accordingly, the waveguide guides optical waves by total internalreflection.

Polarization in a TM (transverse magnetic) transmission mode or a TE(transverse electric) transmission mode are important in photonicintegrated circuits for optical sensing, communication, and signalprocessing, as examples. The principle of the polarizer is to make onepolarization of one type of transmission mode more lossy than anothertype of transmission mode.

Current polarizers require different waveguide dimensions (e.g., heightand width) to transmit in either the TE polarized mode or TMtransmission polarized mode. This adds complexity in device fabrication.Also, constant-radius bend polarizers have junctions with opposite signsof curvature, which leads to mode mismatch and subsequent scattering,elevating the insertion loss of the polarizer.

Moreover, current TM pass polarizers require high aspect ratiowaveguides which are challenging to fabricate and/or require differentdevice layer height for different wavelengths. In addition, gratingbased TM pass polarizers suffer from strong back reflection, where theTE mode lies in the photonic bandgap of the grating.

SUMMARY

In an aspect of the disclosure, a structure comprises: a waveguide stripcomposed of a first type of material and having openings along itslength which are positioned to reflect/scatter propagatingelectromagnetic waves; and grating fin structures on one or both sidesof the waveguide strip which are positioned and structured toreflect/scatter the propagating electromagnetic waves.

In an aspect of the disclosure, a structure comprises: a waveguide stripcomposed of a first type of material and comprising: a straight inputend; a straight output end; and openings filled with a dielectricmaterial along its length between the straight input end and thestraight output end, and which are positioned to reflect/scattertransverse electric (TE) polarization; and grating fin structures on oneor both sides of the waveguide strip which are positioned toreflect/scatter the transverse electric (TE) polarization.

In an aspect of the disclosure, a structure comprises: a strip waveguidefabricated on a silicon on insulator wafer, the strip waveguidecomprising: a straight input and output; a fixed height and width; andperiodic slots along its length between the input and the output, andwhich are filled with a dielectric material; and tapered and periodicgrating fin structures on one or both sides of the waveguide strip, withthe periodic slots being within the grating fin structures; and a bufferregion below the strip waveguide and cladding oxide above the stripwaveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is described in the detailed description whichfollows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the presentdisclosure.

FIG. 1A shows a waveguide TM pass polarizer structure, amongst otherfeatures, and respective fabrication processes in accordance withaspects of the present disclosure.

FIGS. 1B and 1C show performance graphs of the waveguide TM passpolarizer structure of FIG. 1A.

FIG. 2A shows an illustrative waveguide TM pass polarizer structure inaccordance with aspects of the present disclosure.

FIGS. 2B-2D show performance graphs of the waveguide TM pass polarizerstructure of FIG. 2A.

FIG. 3A shows a waveguide TM pass polarizer structure and respectivefabrication processes in accordance with another aspect of the presentdisclosure.

FIGS. 3B-3D show performance graphs of the waveguide TM pass polarizerstructure of FIG. 3A.

DETAILED DESCRIPTION

The present disclosure relates to semiconductor structures and, moreparticularly, to a slot assisted grating based transverse magnetic (TM)pass polarizer and methods of manufacture. More specifically, thepresent disclosure describes a waveguide structure with a TM passpolarizer based on a compact slot assisted grating based TM passpolarizer with very low insertion loss and back reflection.Advantageously, the structures described herein can be scaled to a fewmicrometers (compared to millimeters in previous designs), in additionto eliminating back reflection and having low insertion loss with a highextinction ratio. The structures described herein are also tolerant tofabrication errors, e.g., processing errors.

The waveguide TM pass polarizer structures of the present disclosure canbe manufactured in a number of ways using a number of different tools.In general, though, the methodologies and tools are used to formstructures with dimensions in the micrometer and nanometer scale. Themethodologies, i.e., technologies, employed to manufacture the waveguideTM pass polarizer structures of the present disclosure have been adoptedfrom integrated circuit (IC) technology. For example, the waveguide TMpass polarizer structures are built on wafers and are realized in filmsof material patterned by photolithographic processes on the top of awafer. In particular, the fabrication of the waveguide TM pass polarizerstructures uses three basic building blocks: (i) deposition of thinfilms of material on a substrate, (ii) applying a patterned mask on topof the films by photolithographic imaging, and (iii) etching the filmsselectively to the mask.

FIG. 1A shows a waveguide TM pass polarizer structure, amongst otherfeatures, and respective fabrication processes in accordance withaspects of the present disclosure. More specifically, the structure 10of FIG. 1A includes a waveguide structure (e.g., waveguide strip) 14fabricated on a substrate 12. In embodiments, the substrate 12 can be asilicon on insulator (SOI) substrate or bulk Si, as examples. In an SOIimplementation, the substrate 12 includes a semiconductor materialbonded to an insulator layer. The semiconductor material can be composedof any suitable material including, but not limited to, Si, SiGe, SiGeC,SiC, GaAs, InAs, InP, and other III/V or II/VI compound semiconductors.The insulator layer can be a buried oxide material (BOX); although otherinsulator materials are also contemplated herein. The insulator layer isbonded to a semiconductor wafer.

The waveguide structure 14 preferably has a straight layout whichincludes a straight input end 14 a, a straight output end 14 b and aplurality of openings 14 c along its length. The waveguide structure 14can have a width of W₂. For example, the dimension W₂ can be about 200nm; although other dimensions are also contemplated herein. Thewaveguide structure 14 can be of a fixed height and width (along itslength) for any TM transmission (compared to the requirement ofdifferent device heights for different wavelengths).

The plurality of openings 14 c can be hollow (e.g., filled with air) orfilled with low refractive index dielectric material. In examples, theplurality of openings 14 c can be filled with a SiO₂, Quartz, SiN orother dielectric materials with a refractive index of about 1.2-2.5. Infurther embodiments, the plurality of openings 14 c can be placed in aperiodic (constant) or non-periodic (non-constant) layout, with aninterior dimension of W_(s). In embodiments, and as further describedherein, the dimension can be about 150 nm to about 200 nm; althoughother dimensions are contemplated herein. The spacing between adjacentopenings 14 c (or inside grating fin structures 16) can be about 200 nm;although other dimensions are contemplated herein.

Still referring to FIG. 1A, the openings 14 c can be periodic slots,e.g., holes, gaps, voids, inside grating fin structures 16 (e.g.,aligned with or offset from the grating structures 16). The gratingstructures 16 are preferably between the input end 14 a and the outputend 14 b. The grating fin structures 16 can also be periodic ornon-periodic on one or both sides of the waveguide 14. It should beunderstood by those of skill in the art that any combination of periodicor non-periodic grating fin structures 16 can be used with periodic ornon-periodic openings 14 c. In this way, the grating fin structures 16can be aligned with and/or offset from the openings 14 c.

Also, the grating fin structures 16 can be of a fixed height, e.g., sameheight as the waveguide structure 14; although other dimensions arecontemplated herein. By way of example, the grating structures 16 caninclude a dimension W₁ of about 125 nm (and a length of about 400 nm);although other dimensions are contemplated herein. The grating finstructures 16 and the waveguide structure 14 can have some overlap intheir dimensions. In addition, the grating fin structures 16 can includedifferent shapes, e.g., rectangular, square, semi-spherical, tapered,etc., on one or both sides of the waveguide structure 14.

The waveguide structure 14 and grating fin structures 16 can befabricated of Si, SiN, Poly-Si or any polymer waveguide materials, e.g.,doped Si, doped poly and Ge doped Si materials. Moreover, the waveguidestructure 14 and grating fin structures 16 can be fabricated from InAsor InP. In addition, the waveguide structure 14 and grating finstructures 16 can be fabricated in any material platform that allowsbuilding of planar photonic integrated circuits, e.g., bulk Si. By wayof example, the waveguide structure 14 and grating fin structures 16 canbe fabricated on a silicon on insulator (SOI) wafer, with the buriedoxide (BOX) region (buffer region) below the waveguide structure 14. Anoxide cladding can be provided above the waveguide structure 14 andgrating fin structures 16.

In further embodiments, the grating fin structures 16 can be fabricatedeither with the same materials or different materials from the waveguidestructures 14. For example, the grating fin structures 16 can befabricated with (i) a metallic layer, e.g., gold, (ii) doped (heavily)silicon, (iii) doped (heavily) polysilicon, or (iv) germanium dopedsilicon. The grating fin structures 16 can also be of the same ordifferent height as the waveguide structures 14, having a grating periodvaried for optimizing different wavelengths and/or extinction ratios. Inany of these embodiments, the TM (transverse magnetic) polarized wave(light) and TE (transverse electric) polarized wave (light) can beinputted into the input end 14 a of the waveguide structure 14, with theTE polarized wave (light) being reflected/scattered by the openings 14 cand the grating fin structures 16 hence allowing only the TM polarizedwave (light) to pass through the output end 14 b of the waveguidestructure 14.

The waveguide structure 14 and the grating fin structures 16 can befabricated from the same material or different materials, including thesame material as the substrate 12, e.g., semiconductor material. In thecase that the waveguide structure 14 and the grating fin structures 16are fabricated from the same material as the substrate, these featurescan be fabricated (patterned) by the same conventional lithography andetching processes, e.g., reactive ion etching (RIE). In the case thatthe waveguide structure 14 and the grating fin structures 16 arefabricated from the same materials (but not necessarily the substratematerial), these features can be fabricated by the same conventionaldeposition, e.g., chemical vapor deposition (CVD) processes, followed bythe same lithography and etching processes for patterning purposes. Inthe case that the waveguide structure 14 and the grating fin structures16 are fabricated from different materials, these features can befabricated by the separate deposition, lithography and etchingprocesses.

As should be understood by those of skill in the art, conventionallithography and etching processes include a resist formed over thesubstrate 12, which is then exposed to energy (light) to form a pattern(opening). An etching process with a selective chemistry, e.g., reactiveion etching (RIE), will be used to form the patterns in the substrate 12or other material, resulting in the formation of the waveguide structure14 and the grating fin structures 16. The resist can be removed by aconventional oxygen ashing process or other known stripants.

FIGS. 1B and 1C show performance graphs of the waveguide TM passpolarizer structure of FIG. 1A. More specifically, FIG. 1B shows a TMtransmission and FIG. 1C shows TE transmission and reflection. In bothgraphs, the “x” axis represents wavelength (μm) and the “y” axis ispower (dB). In particular, FIG. 1B shows an approximate TM transmissionof greater than 95%; whereas, FIG. 1C shows an approximate TEtransmission loss of 99%. It should be understood, though, that thesetransmissions and transmission losses can be modulated based on theparticular construction of the waveguide TM pass polarizer structure ofFIG. 1A, e.g., dimensions, materials, etc. Also, it should be understoodthat the results and numbers provided herein are merely an illustrativeexample and should not be considered a limiting feature of the presentdisclosure.

FIG. 2A shows an illustrative waveguide TM pass polarizer structure inaccordance with the present disclosure. In this representation, thegrating fin structures 16 have a dimension of about 125 nm×400 nm, witha length of the waveguide structure 14 between the grating finstructures 16 of about 200 nm. In this representation, a width of theopenings 14 c is about 200 nm (and a length of 400 nm). The grating finstructures 16 and the openings 14 c are periodic, with the grating finstructures 16 and openings 14 c being in alignment. The total length ofthe illustrative waveguide TM pass polarizer structure is about 28 μm,and the area of the opening (closed space) is about 0.06 μm². It shouldbe understood that the numbers provided herein are merely anillustrative example and should not be considered a limiting feature ofthe present disclosure.

FIGS. 2B-2D show performance graphs of the waveguide TM pass polarizerstructure of FIG. 2A. In each of the graphs, the “x” axis representswavelength (μm). In FIG. 2B, the “y” axis is representative of a TMtransmission (dB); whereas, in FIG. 2C the “y” axis is representative ofTE transmission (loss) (dB) and, in FIG. 2D, the “y” axis isrepresentative of TE reflections (dB). As shown in FIG. 2B, the TMtransmission is greater than 90%. In FIG. 2C, the TE transmission lossis greater than 99% and, in FIG. 2D, the TE reflections are less than2%. It should be understood that the results and numbers provided hereinare merely an illustrative example and should not be considered alimiting feature of the present disclosure.

FIG. 3A shows an illustrative waveguide TM pass polarizer structure 10 ain accordance with additional aspects of the present disclosure. In thisrepresentation, the grating fin structures 16 have a tapered profile 16a. In addition, openings 14 c are oval in shape, with tapered corners 14d. The dimensions of the grating fin structures 16 can be about 125nm×400 nm, with a length of the waveguide structure 14 between thegrating fin structures 16 of about 200 nm. In this representation, awidth of the openings 14 c is about 200 nm (and a length of 400 nm). Thegrating fin structures 16 and the openings 14 c are periodic, with thegrating fin structures 16 and openings 14 c being in alignment. Thetotal length of the illustrative waveguide TM pass polarizer structureis about 28 μm. Again, though, it should be understood that the numbersprovided herein are merely an illustrative example and should not beconsidered a limiting feature of the present disclosure.

FIGS. 3B-3D show performance graphs of the waveguide TM pass polarizerstructure of FIG. 3A. In each of the graphs, the “x” axis representswavelength (μm). In FIG. 3B, the “y” axis is representative of TMtransmission (dB); whereas, in FIG. 3C the “y” axis is representative ofTE transmission (loss) (dB) and, in FIG. 3D, the “y” axis isrepresentative of TE reflections (dB). As shown in FIG. 3B, the TMtransmission is greater than 90%. In FIG. 3C, the TE transmission lossis greater than 99% and, in FIG. 3D, the TE reflections are less than3%. Also, it should be understood that the results and numbers providedherein are merely an illustrative example and should not be considered alimiting feature of the present disclosure.

The method(s) as described above is used in the fabrication ofintegrated circuit chips. The resulting integrated circuit chips can bedistributed by the fabricator in raw wafer form (that is, as a singlewafer that has multiple unpackaged chips), as a bare die, or in apackaged form. In the latter case the chip is mounted in a single chippackage (such as a plastic carrier, with leads that are affixed to amotherboard or other higher level carrier) or in a multichip package(such as a ceramic carrier that has either or both surfaceinterconnections or buried interconnections). In any case the chip isthen integrated with other chips, discrete circuit elements, and/orother signal processing devices as part of either (a) an intermediateproduct, such as a motherboard, or (b) an end product. The end productcan be any product that includes integrated circuit chips, ranging fromtoys and other low-end applications to advanced computer products havinga display, a keyboard or other input device, and a central processor.

The descriptions of the various embodiments of the present disclosurehave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed:
 1. A structure, comprising: a waveguide strip composedof a first type of material and having openings along its length whichare positioned to reflect/scatter propagating electromagnetic waves;grating fin structures on one or both sides of the waveguide strip whichare aligned with the openings and positioned and structured toreflect/scatter the propagating electromagnetic waves; a buffer regionof semiconductor on insulator technologies located below the waveguidestrip and grating fin structures; and cladding oxide above the waveguidestrip and the grating fin structures, wherein an interior dimension ofthe openings is W_(s), a width of the waveguide strip is W₂, a width ofthe grating structures is W₁, W_(s) less than or equal to W₂ and W₂ isgreater than W₁, the grating fin structures are composed of a secondtype of material different than the first type of material, and theopenings are filled with a dielectric material that is different thanthe first type of material.
 2. The structure of claim 1, wherein thedielectric material is SiN or SiO₂.
 3. The structure of claim 2, whereinthe waveguide strip is straight and has a fixed height and width alongits length.
 4. The structure of claim 3, wherein the grating finstructures are periodic.
 5. The structure of claim 4, wherein theopenings are periodic or non-periodic.
 6. The structure of claim 3,wherein the grating fin structures are non-periodic.
 7. The structure ofclaim 6, wherein the openings are periodic or non-periodic.
 8. Thestructure of claim 1, wherein the first type of material issemiconductor material.
 9. The structure of claim 1, wherein the gratingfin structures have a tapered profile.
 10. The structure of claim 9,wherein the openings are oval shaped with tapered corners.
 11. Thestructure of claim 1, wherein the propagating electromagnetic waves thatare reflected and scattered are transverse electric (TE) polarization.12. The structure of claim 1, wherein the grating fin structures arecomposed of (i) a metallic layer, (ii) doped silicon, (iii) dopedpolysilicon, or (iv) germanium doped silicon.
 13. The structure of claim12, wherein the grating fin structures are of a different height thanthe waveguide strip.
 14. The structure of claim 13, wherein the gratingfin structures have a tapered profile and the openings are oval inshape, with tapered corners.
 15. A structure, comprising: a waveguidestrip composed of a first type of material and comprising: a straightinput end; a straight output end; and openings filled with a dielectricmaterial of SiN or SiO₂ along its length between the straight input endand the straight output end, and which are positioned to reflect/scattertransverse electric (TE) polarization; grating fin structures on one orboth sides of the waveguide strip which are positioned toreflect/scatter the transverse electric (TE) polarization; a bufferregion of semiconductor on insulator technologies located below thewaveguide strip and grating fin structures; and cladding oxide above thewaveguide strip and the grating fin structures, wherein an interiordimension of the openings is W_(s), a width of the strip waveguide isW₂, a width of the grating structures is W₁, W_(s) less than or equal toW₂ and W₂ is greater than W₁.
 16. The structure of claim 15, wherein thegrating fin structures are periodic and the openings are periodic ornon-periodic.
 17. The structure of claim 15, wherein the grating finstructures are non-periodic and the openings are periodic ornon-periodic.
 18. The structure of claim 15, wherein the grating finstructures have a tapered profile.
 19. The structure of claim 15,wherein the openings are oval shaped with tapered corners.
 20. Astructure comprising: a strip waveguide fabricated on a silicon oninsulator wafer, the strip waveguide comprising: a straight input andoutput; a fixed height and width along its length; and periodic slotsalong its length between the input and the output, and which are filledwith a dielectric material composed of oxide or nitride or quartz basedmaterials; tapered and periodic grating fin structures on one or bothsides of the waveguide strip, with the periodic slots being aligned andwithin the grating fin structures; and a buffer region below the stripwaveguide and cladding oxide above the strip waveguide, wherein aninterior dimension of the openings is W_(s), a width of the stripwaveguide is W₂, a width of the grating structures is W₁, W_(s) lessthan or equal to W₂ and W₂ is greater than W₁.